That's why there are medicals. But that you're a driver now doesn't mean you have to do that until the day you retire.
OK, there are medicals which address whether individual drivers are fit to continue - what happens when they're 62 and struggling with high blood pressure and diabetes (both associated with shift work)? Medicate them and send them back out there? We need to realise that on the whole, it is unlikely for people to be able to continue in front line shift working roles right up to an increased pension age. If now is the time for reform, why not wholesale reform that addresses that as an issue and provides pathways to other roles? Of course you don't have to be a driver until the day you retire but the reality is it is quite a specific skillset and the longer you do it, the harder it is to transition to something else, particularly outside the railway. Ageism in employment isn't exactly something unheard of either.
Again, it is unreasonable to demand railway staff adapt to the modern world in terms of sunday working, pension age, etc without also accepting that employers need to adapt to the modern world and accept and act on what we now know about the impacts of these jobs. This is why you get people digging their heels in over seemingly small issues - if we already have x number of unsociable hours jobs then yes, we're going to refuse a special turn which books on at 0100 and breaks an agreement in some small way. The media would love to spin that as us being unreasonable over shunting time or changing ends or whatever, but when it is a one way street in terms of fatigue and work content that's what happens. It's a wider protest against people who do office jobs in office hours ignoring the reality of what we're being asked to do and the proven health issues associated with shift work.
Two disclaimers: yes, we all know Bob who is happily driving trains at 75 and loves the hours, and yes, plenty of other jobs have unreasonable demands placed on them in terms of shift work.